Conventionally, radiation images such as X-ray images are widely used to diagnose symptoms in medical practice. Particularly, radiation images using an intensifying screen-film system are still used in medical practices worldwide, as an imaging system exhibiting both high reliability and excellent cost performance, as a result of achievement of high sensitivity and high image quality in a long history. However, such image information is so-called analog image information that cannot be subjected to flexible image processing and cannot be instantaneously electrically transmitted, unlike digital image information currently continuing to be developed.
Recent years have seen the appearance of digital radiation detectors typified by radiation detectors of computed radiography (CR) type, flat panel type (flat panel detectors: FPD), and the like. In these radiation detectors, a digital radiation image can be directly acquired and can be directly displayed on an image display device such as a cathode-ray tube panel or a liquid crystal panel, thereby making it unnecessary to form an image on a photographic film. Consequently, such digital radiation detectors, for example, X-ray detectors, reduce the necessity of image formation by a silver salt photographic system, thus greatly improving convenience in diagnostic work in hospitals and clinics.
One of digital technologies relating to X-ray images is computed radiography (CR), which is currently used in medical facilities. However, X-ray images acquired by CR are insufficient in sharpness and spatial resolution as compared to those acquired by screen-film systems such as a silver salt photographic system, and the image quality level thereof is still far from that of screen-film systems. Thus, as another new digital X-ray imaging technology, for example, flat X-ray detectors (Flat Panel Detectors: FPDs) using thin film transistors (TFTs) have been developed.
FPDs as mentioned above convert an X-ray to visible light by using, in principle, a scintillator panel including a phosphor layer made of an X-ray phosphor having properties that convert an applied X-ray to visible light to emit light. Radiography using a low-dose X-ray source requires use of a scintillator panel high in light emission efficiency, which is conversion efficiency from an X-ray to visible light, in order to improve a ratio of signal to noise (a S/N ratio) detected from the scintillator panel. In general, the light emission efficiency of a scintillator panel is determined by the thickness of a phosphor layer and the X-ray absorption coefficient of a phosphor. As the thickness of the phosphor layer increases, emission light generated in the phosphor layer by X-ray irradiation scatters more easily in the scintillator layer, thereby reducing sharpness of an X-ray image that is acquired via the scintillator panel. Accordingly, setting a level of sharpness necessary for image quality of an X-ray image naturally leads to the determination of a limit to the film thickness of the phosphor layer in the scintillator panel.
As used herein, the term “phosphor” is also referred to as scintillator, and the term “phosphor layer” is also referred to as scintillator layer.
Additionally, the shape of the phosphor forming the phosphor layer is also important in obtaining a scintillator panel that can provide X-ray images having high brightness and high sharpness. Many scintillator panels employ a phosphor having a columnar crystal shape as a phosphor forming a scintillator layer, and are usually formed by arranging a plurality of such columnar crystals on a substrate, a support body, or the like. As used herein, each of the columnar crystals forming the scintillator layer has a shape extending perpendicularly to a main surface of the substrate, the support body, or the like so that each columnar crystal can efficiently discharge emission light generated in the scintillator layer in the direction perpendicular to the main surface thereof. Scintillator panels employing such a layout in the scintillator layer can maintain the brightness and sharpness at high levels, as well as high strength in the direction perpendicular to the substrate, the support body, or the like. The phrase “the direction perpendicular to the substrate, the support body, or the like” may be hereinafter referred to as “film thickness direction”.
In recent years, various studies and attempts have been made to focus on a crystal shape of a phosphor forming a scintillator layer.
For example, Patent Literature 1: U.S. Unexamined Patent Application Publication No. 2014-0001366 discloses a radiological image detection apparatus that includes a specific scintillator having a plurality of columnar crystals, in which the scintillator includes cesium iodide and thallium in a specific mole ratio and a rocking curve half-width on a (200) surface of the columnar crystal is equal to or less than 3 degrees. Then, Patent Literature 1 teaches that the above radiological image detection apparatus can obtain high sensitivity by controlling the mole ratio and the rocking curve half-width to be within the specific ranges.
Patent Literature 2: WO 2011/089946 discloses a radiation image conversion panel including a phosphor layer on a substrate, in which a phosphor columnar crystal forming the phosphor layer has an orientation degree, which is based on an X-ray diffraction spectrum of a surface having a constant plane index, within a specific range regardless of the position of the crystal in a layer thickness direction from roots near the substrate to tips of the phosphor columnar crystals in the phosphor layer. Then, Patent Literature 2 teaches that the use of the above structure can provide a radiation image conversion panel that shows improved brightness.
However, even the radiological image detection apparatus disclosed in Patent Literature 1 and the radiation image conversion panel disclosed in Patent Literature 2 still have room for improvement in terms of the sharpness of a radiation image to be obtained, and the like, as will be described below.
The invention described in Patent Literature 2 had the object of providing a radiation image conversion panel showing improved brightness. Then, the invention achieved the object by controlling so that, in the phosphor columnar crystal of the radiation image conversion panel, the orientation degree based on the X-ray diffraction spectrum of the surface having a constant plane index was within a range of from 80 to 100% regardless of the position of the columnar crystal in the layer thickness direction of the phosphor layer.
The invention described in Patent Literature 1 had the object of providing a radiological image detection apparatus having a higher sensitivity than the radiation image conversion panel described in Patent Literature 2 and the like. Then, the invention achieved the object by controlling the mole ratio of the specific components in the scintillator of the above radiological image detection apparatus and the rocking curve half-width on the (200) surface of the scintillator. The invention described in Patent Literature 1 controls the rocking curve half-width to improve the quality of crystal properties of the columnar crystals, thereby improving the sensitivity of the radiological image detection apparatus.
However, according to consideration by the present inventors, even the radiological image detection apparatus disclosed in Patent Literature 1 still has room for improvement in terms of the sharpness of a radiation image to be obtained.
For example, in the radiological image detection apparatus disclosed in Patent Literature 1, a scintillator 120 includes, in addition to columnar crystals 130, non-columnar crystals 130 for causing visible light generated in the columnar crystals 131 to be reflected toward a photoelectric conversion panel 121 (see also FIG. 8C of the present specification), and therefore emission light cannot be generated in the entire scintillator layer. Accordingly, there seems to be a limitation on improvement in the sharpness of a radiation image to be obtained.
In addition, in the radiological image detection apparatus described in Patent Literature 1, the entire columnar crystal portion of the scintillator layer is comprehensively treated to control the rocking curve half-width on the (200) surface of the scintillator layer. However, in terms of the sharpness of a radiation image to be obtained, it seems necessary to more precisely control the rocking curve half-width on the specific plane index.